Immune system

The immune system is the body's biological defense mechanism and protects it against foreign invaders, such as bacteria and viruses. The system is a collection of cells and tissues in the body that protect it against disease-causing organisms. It works by using a simple system in which it distinguishes self (acceptable) from nonself (nonacceptable), and then it attacks and attempts to destroy anything nonself. Nearly all animals, simple and complex, have an immune system based on this self/nonself mechanism.

NATURAL IMMUNITY

The immune system has two different types of defense. The first is called natural immunity and is composed of the basic physical and chemical barriers that every body has at its disposal to fight a foreign invasion. The body's skin is its first line of natural defense since healthy, unbroken skin acts as a physical barrier against microorganisms. If, however, these tiny organisms try to get into the body through normal openings, like the nose and eyes, the body is prepared. These passages are lined with sticky mucus that catches the microorganisms, and with hairlike cilia that sweep them back out of the body. The body also uses secretions, like tears and saliva to protect itself. These secretions contain an enzyme called lysozyme that breaks down the walls of invading bacteria. If after these three defenses, microorganisms still manage to get into the body, the blood contains certain types of white cells called phagocytes that literally swallow up and destroy foreign cells or substances. The body then activates its complement system, which releases proteins that cause an inflammatory response. With this response, the body releases a fluid called histamine that helps fight the invader and results in local swelling.

ACQUIRED IMMUNITY

Sometimes the phagocytes, which make a general attack and attempt to destroy anything detected as foreign, cannot cope with the invader. When this happens, the body's acquired immunity goes into action. If natural immunity is the body's "nonspecific defense," meaning it will attack anything detected as foreign, then acquired immunity is its "specific defense."

Acquired immunity allows the body to "remember" and link past infections to a particular bacteria or virus. The body is then able to respond more quickly the next time it encounters the invader (now called an antigen). Only vertebrates (animals with a backbone) have acquired immunity. Acquired immunity enables the immune system to produce certain types of white cells called antibodies to fight a particular type of pathogen (disease-producing organism). It also enables the immune system to "remember" that pathogen and to respond more quickly the next time it appears. The body produces three types of white blood cells, macrophages, T lymphocytes, and B lymphocytes, that work together and carry out a complex series of events known as the immune response. (Macrophages alert the immune system that specific foreign agents are present.) The primary immune response involves the B-cell lymphocytes producing antibodies that capture and kill the invading antigen. However, when a virus invades a cell, the virus is safe from antibodies, so the T-cell lymphocytes begin what is called cell-mediated immunity. The T-cell is able to recognize any infected cell, and when it does, it kills the cell. Lymphocytes are transported throughout the body via the lymphatic system, a type of secondary circulatory system that acts as a bridge to the immune system.

EDWARD JENNER DEVELOPS IMMUNIZATION

The natural ability of the immune system to be able to recognize a particular antigen is the basis for immunization. Since ancient times, medical observers had noticed that the body seemed to have powers to protect itself and resist disease. In particular, people who had survived a certain infectious disease did not suffer from that disease again during their lifetime. In 1796, the English physician Edward Jenner (1749–1823) discovered that it was possible to make people immune to a disease they never had. First, he gave a person an injection of a dead or weakened microorganism (called a vaccine) that caused a certain disease (like cowpox). The vaccine was not strong enough to give the person cowpox, but still the patient's body would react by producing antibodies against the disease. Jenner found that immunization protected his patients from the dreaded smallpox disease. Eventually, successful methods of immunization were developed against such diseases as diphtheria, whooping cough, mumps, measles, rubella, polio, rabies, anthrax, typhoid fever, typhus, yellow fever, cholera, and the plague.

PAUL EHRLICH

German bacteriologist (a person specializing in the study of bacteria) Paul Ehrlich (1854–1915) is the founder of chemotherapy, which is the use of a chemical substance to treat a disease. He also identified substances that could be used as drugs to destroy bacteria in the body, and made important contributions to the understanding of immunity (the body's natural resistance to a foreign substance).

Paul Ehrlich was born in Strehlin, Silesia (then part of Germany, now part of Poland). His family was educated and well-off, and although young Ehrlich did not do well in school at first, he came to be very interested in both chemistry and biology. He attended German universities and received his medical degree in 1878. Throughout his medical education, Ehrlich was always interested in its chemical aspects, and he became especially interested in the new dyes that were being introduced. Ehrlich was particularly fascinated by the staining (dyeing) of cells and tissues and their reactions to dyes. For his graduate thesis he discovered several practical stains for bacteria and even wrote his thesis on that subject. After working with the famous German bacteriologist Robert Koch (1843–1910) studying tuberculosis, he was appointed a professor at the University of Berlin in 1890.

There he began work with others on the study of immunity, or the body's own defense against disease. The group he joined was trying to find a cure for diphtheria, a childhood respiratory disease that killed many. Ehrlich was searching for a substance that would give immunity against diphtheria by using antitoxins. Antitoxins are antibodies produced by the body's immune system to fight poisons invading the body. An antibody is a special protein in the blood that locks on to a specific foreign substance and kills it. By 1892, Ehrlich had worked out an antitoxin for diphtheria that could be used medically. He obtained the right antitoxin from large animals that had been immunized against diphtheria. He then concentrated and purified it and administered it to 220 children with success. For his work on immunity, Ehrlich later won the 1908 Nobel Prize for Physiology and Medicine. After this achievement, Ehrlich returned to studying dyes and stains, and decided to pursue a fascinating idea. He knew that stains were useful because they colored some cells but not others, thereby making the stained ones stand out. He also knew that a stain would not color a bacterium (plural, bacteria) unless it combined with something in the bacterium. Knowing also that when this happened the bacterium usually died, he theorized that if he could find a dye that stained bacteria but not ordinary cells, then maybe it was also a chemical that killed bacteria without harming the host (the human being). He described such a chemical as a "magic bullet," saying that it would seek and destroy only the invader. Eventually, he did discover one dye, called trypan red, that worked against such diseases as sleeping sickness. Much later, he discovered a dye he named Salvarsan that would kill the microorganism that caused syphilis, a sexually transmitted disease. These two chemicals marked the beginning of modern chemotherapy. Ehrlich proved to be a pioneer not only in the field of immunology, but in the newer field of chemotherapy as well.

HIV AND AIDS

The last few decades of the twentieth century witnessed not a new disease to fight, but the emergence of a disorder of the human immune system itself called AIDS (Acquired Immune Deficiency Syndrome). Infection by this new Human Immunodeficiency Virus (HIV) caused the immune system to collapse, leaving the body defenseless. Specifically, the HIV virus attacked certain T-cells and made them unable to do their job helping B-cells make antibodies. The result was that once a person's natural immune system shut down, they became host to a number of devastating infectious organisms. AIDS is not a single disease but a syndrome of symptoms that are caused by infectious invaders taking advantage of an immune system that cannot function. HIV can remain dormant in the body for some time without producing any signs. There is still no cure for AIDS, although great progress has been made in coping and managing this disease and prolonging the lives of its victims.

Also in the last few decades, biologists have discovered that the immune system can be affected by a person's psychological health or state of mind. Apparently there exists a complex network of nerves, hormones, and brain chemicals that link the immune system to a person's mental state, and it has been demonstrated that extreme psychological stress can suppress the immune system and accelerate certain diseases. Recent immunological research indicates that the mind/body connection is more significant than previously thought.

[See alsoAIDS; Antibody and Antigen; Immunization; Lymphatic System; Vaccine ]